Both sketch and invision are easy to use and quick to setup. You can create prototypes for different device types. There are many plugins available that allow you (or your designers) to work even faster. For bootstrapping the one month free trial of sketch and the one prototype for free version of invision offer a great option to build a prototype without any cost.

We use Sketch at Olio Digital Labs, and it is, hands down, way faster and easier to use than Photoshop. You can easily switch from wireframes to full fidelity designs, export CSS, and there are new, useful plugins being released frequently.

Atomic is one of my favorite tools. It's quite flexible and will work for low- and high-fidelity prototyping. It's simple to get started as a non-designer, as Atomic recently released a series of videos. You can collaborate with others on your team in real-time, so you can build prototypes quickly. Highly recommend Atomic!

Atomic is my go-to solution for ideating, creating something to test with customers, and communicating with developers. You can get an idea of how I use prototyping tools in general here. It includes an overview of many tools in this space.

XD is extremely quick and intuitive use, with both design and prototyping integrated into one. The share option immediately creates links that enables you to just link, make comments about your prototype. It's an absolute favourite!!

Cliche: content is king.
It's true.
Even if I started my career as a mega obsessed UI designer... I now focus on substance rather than layout.
Simplicity is key. It's the words that matter.
My prototypes or MVPs are simple blog posts / pages on a Wordpress site.
At some point, layouts and some heavy custom styling might make sense... that takes a while.